1. Field of the Invention
The present invention relates to an insulated gate type semiconductor device, more particularly to a semiconductor device including a lateral power MOS field effect transistor which is used for supplying a high power output.
2. Description of the Related Art
While lateral MOS devices which allow a current to flow along a principal surface of a semiconductor substrate exhibits an advantage of a possibility to form a large number of elements on a single substrate, they require a wide device area. Accordingly, as a method to solve this problem, an employment of a gate electrode having a trench structure has been proposed.
FIG. 25 is a schematic cross-section showing an example of a semiconductor device to which the lateral MOS field effect transistor (hereinafter referred to as an MOS FET) structure having a trench gate electrode 60 is applied. The semiconductor device shown in FIG. 25 includes a silicon substrate 10; a drift region 14 containing n-type impurities, formed on the silicon substrate 10; a p-type body region 16 formed by diffusing p-type impurities in the principal surface of the drift region 14; and a source region 18 formed by selectively diffusing n-type impurities in the surface of the body region 16. At the surface of the drift region 14, an n-type drain region 20 is formed so as to be apart from the body region 16. A source electrode 44 is formed on top surface of the source region 18 and body region 16, and a drain electrode 46 is formed on the surface of the drain region 20.
The employment of such trench gate makes it possible to form a gate in a depth direction of the semiconductor substrate, so that an area per a unit cell can be smaller compared to semiconductor devices employing a planar gate structure.
However, the lateral MOS devices having the trench gate as shown in FIG. 25 entails the following problems. Specifically, in this lateral MOS device, a breakdown voltage of the device is generally determined by the pn junction formed between the body region 16 and the drain region 20 when 0V is applied to the gate. However, in the structure having the trench gate which is formed at the surface of the semiconductor substrate, upon application of a voltage between the source and the drain, the high electric field occurs at the bottom corner of the trench gate 60, and this high electric field may break the element. To prevent the breakdown phenomena due to such high electric field, a sufficiently deep body junction is required, which makes a high breakdown voltage. However, this leads to widening the body region 16 also in the lateral direction, so that an area occupied by the device increases. Therefore, a reduction of the device cell size may be disturbed and a difficulty to lower an on-resistance may be brought about.